1. Field of the Invention
This invention relates to improvements in fault detection methods and circuits, and more particularly to improvements in methods and circuits for detecting a partially or fully faulty recirculation diode in a switched mode type power supply, and to improvements in switched mode power supply circuits.
2. Relevant Background
Although the invention herein may be applicable to power supply circuits in general, it is described particularly with reference to a switched mode power supply. A switched mode power supply is a power supply that has a switching transistor in series with a recirculation diode between a supply voltage and a reference potential, or ground. A filter may be connected between the transistor and diode to an output terminal, and frequently comprises an inductor in series between the transistor-diode junction and the output terminal, and at least one capacitor between the series path of the inductor and ground. The switching transistor is switched on and off under control, for example, of a microprocessor, microcontroller, driver IC, timer, or other such control device.
The switching transistor may be a MOSFET or other suitable switching device, and generally, in the case of an FET device, a resistor is connected between the source and gate to provide a reference voltage to the switching device. A second, control transistor may be connected to control the gate of the switching transistor. Such control transistor also may be a MOSFET device, and may be connected to switchably apply a supply voltage directly to the gate of the switching transistor in response to control signals from the microprocessor, or other control device.
In operation, the supply voltage is switchably developed across the recirculation diode by the switching transistor and applied to the output filter. When the switching transistor is on, the circuit is considered to be in an on-state, with the voltage across the recirculation diode being directly applied to the output filter. On the other hand, when the switching transistor is not conducting, the circuit is considered to be in an off-state. In the off-state, the currents that are set up in the inductance of the output filter are maintained by the recirculation diode, which allows current to be drawn from the reference potential, or ground, to the inductance.
In many applications, such as in automotive environments or the like, the supply voltages at the input to the circuit may have considerable voltage swings. Likewise, the loads attached to the output terminal of the circuits may draw excessive currents. If these voltage swings or excessive currents are not monitored, it may be possible for high currents to exist that may damage or destroy the components of the circuit, such as the switching transistor, the recirculation diode, or the components of the output filter. To protect against such high currents, switched mode power supply circuits are often provided with over-current detection circuits. Thus, if an over-current state arises in the circuit when it is in an on-state, for example, caused by an over current in the recirculation diode or at the output, protection circuitry is activated to protect the switching transistor from being damaged or destroyed.
Typical protection circuitry that may be used in switched-mode power supplies has a resistor in series with the switching transistor, and an amplifier connected to detect the voltage developed across the resistor. In an over-current conduction in the on-state of the power supply circuit, the amplifier detects a particular threshold voltage existing across the resistor, and produces an output warning. The amplifier is normally disabled during the off-state (or recirculation state) of the circuit. The switching transistor also is normally in an off state, with its gate voltage clamped to its source voltage.
If the recirculation diode partially fails, an abnormal recirculation occurs. In such condition, the diode can be thought of as a resistance in series with an ideal diode. The abnormal recirculation voltage is seen as an increased voltage across the resistance, resulting in increased power dissipation. This increased dissipation reduces efficiency, but would not be detected by the over-current detection circuit as a fault. What is needed, therefore, is a circuit that can effectively detect conditions in which the recirculation diode has partially or fully failed.